How to Use a Star Ground for Mixed Signal Grounding
Grounding electrical systems can be a complicated topic that stymies even the most experienced designers. For power distribution systems, the requirements tend to be rather clear; don't create multiple ground points separated by sufficient resistance to cause a ground loop. In mixed-signal PCBs, whether they run at low or high speed, ground is as much about establishing a consistent reference potential as it is about preventing electromagnetic interference (EMI) from passing between different circuit blocks. You generally have two options for a system with a single supply point: ground planes or star grounding.
So how do we implement the digital ground and analog ground regions with either method in a mixed signal PCB? There are a number of posts about this topic in this blog, and it's easy to take design guidelines out of context. Grounding in mixed signal PCBs is often discussed in the context of high speed digital signals and moderate to high frequency analog signals. However, understanding what happens at lower speeds with both methods can help you plan out a return path to prevent EMI.
The Mixed Signal Grounding Problem
Grounding mixed signal PCBs is a complex issue with many problems and solutions. You need to keep your analog and digital signals separate to reduce EMI, but they also need to be pulled together to a consistent ground potential. In other words, the "0 V" you use to measure between the digital section of your board and ground should be the same 0 V for the analog section. When the two sections are not pulled to the same reference potential, a return path from a high current component can create a large voltage fluctuation for a low current component. If this happens when, say, a digital return current passes near the ground point of an analog component, a slight voltage fluctuation can impose noise on the analog component's output. Even when there are no outside noise sources interfering with the system, noise will appear in the analog signal.
The above description should show you that the main EMI problem within mixed signal boards is digital circuits. Digital chips can emit strong noise when they switch between digital states. When paired with analog circuits, EMI from digital signals will often find itself imposing noise on sensitive analog signals. That’s why you should generally try to keep these two sections of your system separated.
This leads us to the two options for mixed signal grounding: star grounding vs. analog and digital ground planes. Each of these has its merits in different situations, but they need to designed properly to prevent noise problems.
What is Star Grounding?
A star ground is a single point that connects analog and digital ground planes or traces. A star ground is not literally in the shape of a star, but the idea is that each connection is made at a single central point. This is shown in the image below, where the ground connection is at the central point.
In star grounding, all analog and digital ground connections should terminate at your star ground.
The topology in star grounding is simple: connect the different digital and analog ground planes or traces back to a single point (the power supply return), but how can we use this to prevent noise in a mixed signal PCB? Let's look at how ground planes work as this will help illustrate when to use star grounding.
Analog and Digital Ground Planes
In a multilayer PCB, or on the back side of a single layer PCB, large copper regions can be placed on a layer and grounded. The goal is to provide a large region with consistent ground potential, which can then be used to provide a short ground return path for your components. This also provides a side benefit of shielding against external EMI. The typical strategy is not to separate the ground plane into two physically separated planes. Instead, you should try to arrange the digital ground in one area, and the analog ground in a different area, both of which occupy the same plane (see below):
In effect, a large ground plane acts just like a really big star ground when there is only one power supply point on the board! You're connecting the ground plane back to a single point on the board (the power supply return), and all other ground paths connect to the ground plane. So what exactly is the difference between the two?
The difference lies in how return currents travel in your mixed signal PCB. As was mentioned above, the return current from the digital section can induce noise in the analog section. It can happen in the other direction as well, but this is less noticeable. When the digital and analog signals operate at respectively high speeds (less than ~10 ns rise time) or high frequencies (above ~MHz), the return path follows closer to the path of least reactance, which tends to be closer to the components. At lower speeds/frequencies, the return path follows closer to a straight line back to the power supply return.
This means that, at lower frequencies, digital return currents from the digital section of the ground plane are at risk of travelling near analog components. This is what allows noise to be introduced into the analog components. To learn more about the return path at different frequencies, take a look at this article.
Separating analog and digital circuits creates another issue--a floating ground. Your analog and digital chips all need to be referenced to the same ground plane in order to operate properly, but separating the analog and digital ground planes can create potential difference between the two regions. If you connect your separate analog and digital ground planes wherever you want, you can create ground loops. Large ground plane loops will act like antennas and radiate EMI to other regions of your board, and possibly outside of your device. A star grounding for PCB mixed signal grounding allows you bring both your analog and digital circuits together at a single point. This reduces the possibility of ground loops and EMI radiation.
When to Use a Star Ground
Because the return currents from low speed/low frequency signals can pass between digital and analog sections of a large ground plane, these signals need to be isolated from each other. The star grounding method, which is much easier for mixed signal boards, is to split up a ground plane and connect them together at a single point. You could also totally remove the ground plane and just route grounds with large traces or ground pour. You'll have much more control over return current paths as you are defining the regions where they can exist directly on the board. It's easy to separate analog and digital components and signals into different areas with this strategy.
When designing star grounding, your analog and digital circuits should interact as little as possible. Here are some tips to help you out:
- Use separate power supply rails: Route these rails separately and connect them only at the star point.
- Use a single ground point: This will ensure that ground loop areas are minimized and that analog and digital signals don’t interfere with each other.
- Route different return paths far from each other when possible: Creative routing is the key to star grounding. You can even use ground pour regions on the surface layer next to your components if you like as this will provide more shielding between circuit blocks, although be careful as this can create assembly problems with SMD components (e.g., tombstoning).
The perfect time to use star grounding is when you have a single PCB with a single ADC/DAC on it. In that kind of system, you can use your star ground point for your analog-to-digital converter. Then, connect your analog and digital ground planes on either side of the converter at that same point as well. This will help you greatly reduce noise in your mixed signal circuit and make grounding a breeze.
If you’re designing a multi-card system, or are using more than one converter, a star ground might not work for you. The problem is that you’ll have more than one star point, creating the potential for lots of ground loops. The whole idea of a star ground is that everything is connected at one point. If you can’t connect everything at one point, don’t even try.
Keep your circuit well grounded.
If you don’t properly ground your mixed signal circuit to the ground plane correctly, things will get noisy. A star ground can help reduce EMI on your PCB and keep everything in its right place. However, you should take care to only use a star ground in systems with a single converter. In summary:
- Star grounding is fine for lower speed/frequency systems. It is even preferred for things like audio systems, where low speed/frequency signals can easily interfere in the ground plane.
- Ground planes are standard in high speed/frequency systems to provide isolation between different layers and suppress crosstalk.
- In both cases, carefully plan your layout and return paths to minimize interference between analog and digital ground regions.
- Once you get into more complex systems, creating a single star grounding point becomes almost impossible, and you'll have to work with ground planes on a multilayer PCB.
CircuitStudio® can make your work life easier with a great selection of advanced features to help you design a board with minimal EMI. You'll have access to a set of CAD tools for PCB layout, routing, and stackup design.
Have more questions about star grounds? Call an expert at Altium Designer.